Gas chromatography includes three basic stages; an injection stage where a sample is injected into a separation column located within a gas chromatograph, a separation stage in which a pressurized carrier gas forces the sample to flow through the column and is then separated by the interaction between the sample and a liquid phase bound to the inner walls of the column, and a detection stage where the elution from the column of the separated sample is ascertained. The separation stage is relatively long, and may take twenty minutes or more for the separation of very complex molecules. It is known that increasing flow rate and/or temperature within the gas chromatograph decreases the amount of time required to separate a sample, however such action may reduce efficiency and resolution of the chromatography.
U.S. Pat. No. 4,994,096 discloses how efficiency and resolution can be enhanced by regulating the flow rate and temperature in accordance to the mass flow rate of the carrier gas. FIG. 1 illustrates a gas chromatograph 10 having an integrated pressure programmer and includes a column 18 positioned within oven 24. In order to perform a chromatographic separation of a given sample compound, the sample is injected into a pressurized carrier gas by means of an injection port 12. Valve 14 serves to control the pressure of the carrier gas in response to a control signal 15. A pressure transducer 16 generates a pressure information signal representative of the carrier gas provided to injection port 12. A heating unit 26 provides heat to the oven in response to a control signal 27 and a temperature sensor 28 generated by a computer 40. The carrier gas/sample combination passing through column 18 is exposed to a temperature profile resulting from the operation of the heater 26 within oven 24. Mass flow controller 22 is employed for controlling the amount of carrier provided to the column 18. As components exit column 18, they are detected by detector 30. FIG. 2 illustrates the electronic controls including keypad 38, computer 40 and controller 42. Computer 40 includes a central processing unit and all associated peripheral devices, such as random access memories, read-only memories, input/output isolation devices, clocks and other related electronic components.
An analyst attempting to separate and identify certain compounds typically refers to compilations of chromatographic methods to obtain a method which has the greatest potential for separating and identifying certain compounds. One such compilation is entitled "Analytical Solutions--a Collection of Chromatograms from Hewlett-Packard". This compilation contains a collection of over 400 capillary column chromatographic methods developed by a variety of Hewlett-Packard field and factory personnel over a period of ten years. FIG. 3 illustrates one chromatographic method entitled "Drug Standard (2)" which is employed for identifying known drugs and highlights which column parameters and which operating parameters are required to replicate this method. Column parameters include: column length (1), column inside diameter (d) and stationary phase (thickness (d.sub.f) and type). Operating parameters include: carrier gas (type, pressure and/or flow rate) and oven temperature (including program ramp rates). A sample chromatogram highlighting the time at which known compounds will elute from the column is also illustrated, thus making it possible to tentatively identify these compounds based on elution order and time.
It is generally not possible to change only one of the column or operating parameters without affecting the elution time of a given compound, resolution and in many cases the elution order. However, the vast number of column types and sizes makes it quite conceivable that an analyst may be unable to practice a published method for lack of a specified column size. Therefore, an analyst may wish to modify a method to work with a column not specified in a desired chromatographic method, or use smaller columns to increase the speed of separation or larger columns to increase sample capacity. Such modifications require a tremendous amount of time to implement as chromatographic theory, intuition and empirical experimentation are required to ascertain new operating parameters which will provide substantially the same chromatographic output as with the existing column. Once the new operating parameters are ascertained, they must be manually inputted into the gas chromatograph. Again, once a method has been developed, it is generally not possible to change any of the column or operating parameters without changing the chromatographic output.
It is generally known that the speed of a chromatographic process is inversely proportional to the resolution. When the chromatographic output contains well resolved peaks, it is possible to make a tradeoff between speed and resolution. Faster analysis time in a chromatographic process may be accomplished by implementing one or more of the following: a shorter column, a column with a smaller inside diameter (i.d.), a thinner liquid phase film, a faster temperature program rate, or a higher carrier gas linear velocity. An analyst will typically first try changing the temperature program rate and/or the carrier gas velocity. Since the magnitude of the change is determined empirically, there is no quantitative way of achieving a calculated change in speed. Additionally, such a change will typically affect the chromatographic output.
It is known that smaller diameter columns provide for faster chromatography, however, smaller columns also require smaller injected sample sizes, as well as injectors, inlets and detectors which have a correspondingly faster rate. It is therefore desirable to translate a known chromatographic method to provide for reduced sample sizes such that improved speed (decreased runtime) can be achieved without changing the elution order or resolution.